When a company needs money to build its business, it issues stock so that investors, like you, can invest your money in return for a share in the company.
A share is simply a division of a company's stock into equal portions that can be held by shareholders. By owning a share you are owning a piece of a company. Investors choose to own shares in order to make profit, grow their assets and because they believe in the company.
Fractional shares allow you to buy an even smaller portion of a share, such that you don't need to pay the full price of a single unit of a share, which can be expensive. With fractional shares, you could invest just $5 into a stock and earn a fraction of that company's share.